Calls executed in the same "flash frame" (i.e. in the same execution path) are guaranteed to be processed in the same order on the server no matter what channel you are using. RTMP provides an order guarantee in general since everything gets queued up in the order in which they are executed and put into the socket in that order and then processed synchronously (one at a time) on the server. There is a small risk of getting out of order calls when you are using HTTP based channels and you make the calls in separate flash frames. In this case, each call may not be batched into the same HTTP request... instead they are executed independently by the player and the browser may open up 2 or more sockets at the same time. That means there is not a 100% guarantee of the order of execution on the server.
This is something we hope to address in future versions of LCDS - i.e. add an option to make sure things are received and processed in the right order on the server .